The present invention relates to a method of feeding solid material into a pressurized space in a pressurized combustion or gasification plant.
In pressurized combustion or gasification plants, fuel and other material to be introduced into the combustion chamber have to be fed from the atmospheric pressure of a storage container to a state of high pressure within the combustion chamber. The pressure of the combustion chamber varies from 5 to 100 bar. Conventional feeding means oftentimes fail in such high pressure feeding.
Fuel may be introduced into a pressurized space by means of a briquette press. However, the briquettes have to be crushed, for example, with a grinder before they reach the combustion or gasification stage. The briquettes easily become too hard, which impedes the grinding. Furthermore, electric power consumption of the briquette press is high.
The best-known and most common way of feeding fuel into a pressurized space is feeding with a pressurized rotary feeder. The rotary feeders, however, involve certain problems such as poor tightness, risk of arching, and restrictions on the pressure which can be achieved with such feeders. In rotary feeders, the sluice chamber is alternately connected to a fuel storage container and a pressurized fuel chamber. The pressurized space is supplied with fuel and the fuel amount dosed in accordance with the volume of the sluice chamber. To obtain the desired fuel feed, the sluice chamber has to be relatively large, which raises the cost of the equipment.
In rotary feeders, e.g. pocket feeders, the infeed material easily arches. In such cases, poor emptying of the pocket constitutes a problem when the feeder moves to a discharge position. Similar problems with arching and uneven emptying have been experienced with storage silos. In addition, empty pockets of rotary feeders always convey some pressurized gas out of the system, and handling of such escaped gas is difficult. Furthermore, as the loss of pressurized gas increases, the cost of pressurization increases.
It is important that the feeders be gas-tight. Feeding of fuel into a pressurized space has to be facilitated without any pressure being released or harmful gases being leaked out through the feeder. Light or loose fuel, e.g. in screw feeders, is not capable of forming a tight barrier against hot gases escaping from a pressurized space. Thus, such gases possibly flowing back may ignite the fuel already in the fuel silo.
By means of plug screw feeders, fuel may be compressed to some extent so as to facilitate feeding of fuel to a pressure of about 4 bar. In plug screw feeders, the fuel may, however, form a plug which partly or completely clogs the feeding means. If the material to be fed is e.g. peat, the moisture content of which is higher than 65%, no plug is formed, but the material is decomposed and the feeding fails. Feeding to a pressure in excess of 10 bar by means of a plug screw is unsuccessful even though the fuel is dryer. If the moisture of, for example, milled peat is about 50 to 75%, feeding thereof to a pressurized combustion or gasification chamber by earlier known means is generally unsuccessful, and the electric power consumption of the plug screw is also high.
Finnish patent application 864545 teaches a pneumatic transport system for feeding fuel into a pressurized space. The feeding means comprises an enclosed vessel with a rotor and at least one funnel-shaped material chamber disposed in said vessel. The rotor is movable with a drive means. The upper section of the vessel and rotor is provided with a fuel inlet and the lower section with a fuel outlet. Fuel is discharged from the material chamber by means of pressurized gas. For that purpose, pressurized gas is supplied to the outlet duct of the material chamber. Fuel easily arches in funnel-shaped material chambers. Arching prevents complete emptying of the material chamber, thereby causing uneven fuel feed. Some pressurized gas remains in the material chamber after the fuel has been discharged therefrom. Even though the pressurized gas is discharged, some of this gas remains in the material chamber and, in refilling, such gas is harmful as it may prevent even distribution of the fuel in the material chamber.
An object of the present invention is to provide an improved method and apparatus for feeding solid material into a pressurized space, even when the pressure difference is substantial.
Another object of the invention is to provide a method of feeding moist solids.
A further object of the invention is to provide a method and apparatus for introducing loose fuel, such as peat or other bio-fuel, into a pressurized space.
A still further object of the invention is to provide an improved feed apparatus where the feed-disturbing arching phenomenon has been eliminated or minimized.
To meet the objects mentioned hereinabove, the method according to the present invention is characterized in that the chamber volume of a sluice chamber is enlarged in the loading stage and reduced in the unloading stage.
The feed apparatus according to the invention is further characterized in that the sluice chamber is cylindrical and that it is provided with a piston having substantially the same diameter as the cylinder and being capable of adjusting the chamber volume of the sluice chamber.
In accordance with a preferred embodiment of the invention, the feed apparatus comprises two cylindrical sluice chambers disposed in a cylindrical vessel. Both cylindrical sluice chambers are disposed at equal distances from the center of the vessel encasing the sluice chamber. As the vessel rotates around its center, both sluice chambers thus move at equal distances from the center of the vessel.
The invention is also suitable for a feed apparatus in which more than two sluice chambers are disposed in a vessel. However, the number of sluice chambers is preferably even, the sluice chambers being disposed in pairs at equal distances from the axis of the vessel. When one sluice chamber is being filled, the other is being emptied at the same time, the opening of the sluice chamber alternately communicating with the inlet and the outlet.
In accordance with a preferred embodiment of the invention, the piston having substantially the same diameter as the inner diameter of the cylinder is, in the beginning of the loading stage, pushed in the cylinder to its furthest position so that the volume of the sluice chamber is at its minimum and practically zero. During the loading stage, the piston moves in the cylinder so as to enlarge the volume of the sluice chamber. The piston contributes to the filling of the sluice chamber with solid material by causing a suction effect from the inlet channel to the sluice chamber. Preferably, the sluice chambers are disposed vertically within the vessel so that gravity also contributes to the filling of the sluice chamber with solid material.
As soon as the sluice chamber has been filled with a suitable amount of solid material, the vessel moves around its axis, for example 180.degree. . A smaller or larger movement is also possible, depending on the ratio of the cross surface of the sluice chamber to the cross surface of the vessel. The rotational movement is carried out over a time period sufficient to permit, during that movement, the pressure in the sluice chamber to be raised by pressure feeding means substantially to the level of or higher than the pressure of the pressure channel. During the rotational movement, the sluice chamber opening is closed, sliding tightly along the covering plate, in order to prevent gas or solid material from escaping from the sluice chamber.
The rotational movement of the vessel ends as soon as the sluice chamber opening reaches the outlet in the covering plate. During the unloading of the sluice chamber, the piston moves within the cylinder towards the outlet, the solid material simultaneously being conveyed from the sluice chamber to the pressure channel. Transfer of the solid material may be brought about by either the piston effect or by raising the pressure in the sluice chamber higher than that in the pressure channel, or by a combination of the two.
Thus, in accordance with an exemplary embodiment of the invention, feeding fuel into a pressurized space is effected in stages by a feed apparatus comprising an inlet, sluice chamber, and outlet, and the feeding is carried out in such a manner that
in a loading stage, solid material is conveyed from an inlet channel via the inlet into the sluice chamber,
in a first transfer stage, the sluice chamber is shifted respective to the inlet and the outlet, or the inlet and the outlet are shifted respective to the sluice chamber so as to close the inlet and to open the outlet,
in an unloading stage, solid material is conveyed from the sluice chamber via the outlet into a pressurized channel, and
in a second transfer stage, the sluice chamber is shifted respective to the inlet and the outlet, or the inlet and the outlet are shifted respective to the sluice chamber so as to open the inlet and to close the outlet. The invention is especially suitable for feeding bio-fuel, such as bark and peat, into a pressurized space. It may also be employed in feeding of moist fuel.
The invention also relates to an apparatus for feeding solid material in a pressurized combustion or gasification plant from an inlet channel into a pressurized channel, the feed apparatus in an exemplary embodiment comprising
a vessel which is rotatable around its axis,
a covering plate, fixedly positioned on an end wall of the vessel and having at least one inlet communicating with the inlet channel and/or at least one outlet communicating with the pressure channel, and
a sluice chamber disposed in the vessel so as to provide a tight connection between the opening in the sluice chamber and the covering plate.
A great advantage of the method and apparatus according to the invention is that either peat or some other light bio-fuel may be fed into a pressurized, fluidized bed gasifier or combustion plant. Feeding is undisturbed even if the pressure difference is high, i.e. over 4 bar or even over 20 bar. The method according to the invention is suitable e.g. when feeding fuel to a pressurized combustion plant or a gasifier.
In the feeding method in accordance with the invention, the risk of arching of the fuel is eliminated or minimized because the volume of the sluice chamber is changed, thus bringing about movement of the fuel.
In the feeding method and apparatus, caking of the fuel does not occur, which is important as to further treatment of the fuel.
It is, of course, important that the covering plate secures the tightness of the sluice chamber during the feed. Solid material cannot escape from the feed system. Neither hot nor poisonous gases leak backwards from the pressurized space, for example, to the space filled with fuel.
In conventional pressurizing systems for solid fuels, such as Lock-hopper equipment and rotary feeders, it is typical that gas leaks from the pressure side to such an extent that it affects the process yield. The leak has to be compensated by feeding a corresponding amount of gas to the pressurized space by a gas compressor. If the gas to be replaced is, for example, nitrogen, as is usual in connection with gasifying processes, the cost incurred may be considerable. In connection with an apparatus according to the invention, such leak is very small because gas leaks occur practically through sealing means only.
Other objects and advantages of the invention will become apparent from the detailed description which follows.